Over the last few years, the number of wind turbines and wind farms connected to the electricity grid has increased considerably. For this reason, grid operators have set stricter demands for these machines, specifying a series of operating requirements to be met in the event of symmetrical and asymmetrical faults in order to avoid the disconnection of the asynchronous generators and grid destabilization.
It is important to observe that doubly fed machines exhibit different behaviour, depending on whether the fault is asymmetrical or symmetrical. During grid voltage dips, the converter over-current protection causes the converter to disconnect since it cannot control the current imposed at the rotor as a result of the short circuit at the stator. However, the said disconnection is not sufficient to protect the system given the fact that the current flows through the free diodes of the converter, causing the BUS voltage of intermediate circuit (3) to increase and putting the converter component parts at risk. Therefore, in order to protect the converter, the rotor is short-circuited and the generator is disconnected from the grid.
With regard to asymmetrical faults, the problem is aggravated as a result of the permanent appearance of current oscillations that make it impossible to regulate the generator power, as a result of the activation of the converter protective devices. It is necessary for control mechanisms to be introduced into the regulation system in order to continue to regulate generator (7) in the event of an asymmetrical fault. Without these mechanisms, which are the object of this invention, generator (7) is finally decoupled from the grid. These control mechanisms, which are the object of the invention, are described in section 7 (Systems control during asymmetrical and symmetrical faults).
Therefore, the control and regulation system must take the fault typology into account. At present, there are various solutions for dealing with the problem created by symmetrical faults, some of which are shown in documents WO 03/065567, WO 2004/067958, WO 2004/091085 and WO 2005/015730. However, there is no all-inclusive solution which, based on the requirements of the different codes and regulations, resolves the problems created by asymmetrical faults (two-phase or single phase).